Good for you, Cabinboy!!!
Darren, I think if you are serious about Cabin Crew, then you should try and get some experience in Customer Service/Hospitality, as has already been said. Even a job as a waiter will give you some idea of what dealing with different people involves and how to cope with their demands.
Each airline has different specific requirements, but the list Cabinboy gave you covers most of the essential attributes they look for. From a personal point of view, I would put a lot of emphasis on learning as many languages as you can. Having knowledge of a few languages will always make you more attractive to an employer.
I would also stay at school as long as possible. Unfortunately, I don't know exactly how it works in the UK, but leaving school at 15 or 16 seems a little young.
Good luck!